1. Field of the Invention
The present invention relates to a method and apparatus for recording optical information, and more particularly to a method and apparatus for recording optical information capable of precisely controlling emission of laser light for recording.
2. Discussion of the Background
According to a recent spread usage of multimedia, optical readable disks and their associated recording and playback apparatus have been developed rapidly in addition to an increase use of those dedicated to playback mediums such as music CDs, CD-ROMs and DVD-ROMs. More specifically, write-once optical disks using a dye recording medium and phase-change optical disks using phase-change medium have been developed. In these apparatuses, a semiconductor laser light (LD) is commonly used to playback and record.
FIG. 1 illustrates a signal set of a channel clock, recording data and a LD waveform generated through a recording operation performed by a background optical information recording apparatus.
One example recording waveform for applying power to the LD to record information on the dye recording medium is a single pulse waveform created based on eight-to-fourteen Modulation (EFM), as shown in FIG. 1. The single pulse waveform, however, is apt to generate distortions such that recorded marks are rounded due to accumulated heat during a time of recording. Consequently, the recording quality is decreased by these unsharpened recorded marks.
FIG. 2 is an illustration of another signal set of a channel clock, a modulation signal and a light waveform to explain advanced waveforms. This signal set represents one example waveform strategy for LD emission to record information on the dye recording medium. Such waveform strategy uses multi pulses based on recording data with eight-to-sixteen (8-16) modulation code to form marks. Recently, faster linear velocity for recording has been required and, as a result, it becomes increasingly difficult to make the LD responds to such a high speed operation as the multi pulse laser light emission. For this reason, a so-called “Castle write Strategy” waveform is proposed. The so-called “Castle write Strategy” waveform includes peak levels at both rising and failing edges of a pulse to emphasize the mark edges. The peak levels are generated by superimposing a power having a predetermined level on a single pulse at its rising and failing edges. A resultant superimposed peak level is called as a boost level.
FIG. 3 is an illustration to explain changes of the emission power due to changes of the LD drive current. When the dye recording medium and the phase-change optical medium are recorded, a control of a light emission power is precisely needed because characteristics of drive current versus light emission power (i.e., an I-L curve) may drift with heat generated by the LD itself. To make the light emission power stable, a method called automatic power control (APC) is widely used. In the APC, a part of a laser beam emitted from the LD is detected by a photo detector (PD) and then the LD drive current is controlled by a monitor current generated by the PD, which is proportional to the LD emission power.
In a playback operation, a high frequency current is generally superimposed on the LD drive current to reduce electric noises. In this case, a simple configuration of a feedback loop circuit of a low band path can be used to implement the APC because the high frequency current is substantially constant from a viewpoint of a DC current. However, the LD emission power for recording, which changes at an extremely high frequency to form mark/space, needs to be adjusted in a more accurate manner, when the APC is implemented in a recording operation. Therefore, further improvements on the control sequence are strongly requested. The LD drive current can be controlled to some degree with a relatively simple circuit configuration using a fact that a digital sum value (DSV) becomes zero in CDs and DVDs. However, it is almost not possible to precisely control the recording power by such a relatively simple circuit configuration.
For example, when a longest data length, i.e., 11-T mark and space is recorded on a CD-R medium with the writing strategy shown in FIG. 1, a sample hold circuit may be used to keep emission power. Using the sample hold circuit, a few MHz of frequency band may be enough to control even if a rotating speed of the recording medium increases up to four times faster. Thus, the relatively cheap circuit may be able to control the recording power.
During the recording operation, a laser wave length and a sensitivity at the position of the recording disk surface may change due to the increase of temperature in the apparatus and the changes affect suitable recording power of the LD. Therefore, a so-called Running OPC (Running Optimum Power Calibration) is used to adjust recording power in accordance with a level of reflected light (i.e., RF signal) detected during the recording operation.
FIG. 4 is an illustration to explain a problem caused when the reflected light is detected. A reflection rate of the reflected light at a recorded spot rapidly decreases soon after the spot is exposed by the laser beam. The RF signal is high at the rising edge of the pulse and is decreasing to low at falling edge of the pulse, as shown in FIG. 4. To detect the RF signal properly, sampling may be performed at the last part of the pulse where the pulse becomes relatively stable.
However, in a case of a high speed recording on the dye type DVD medium with the so-called “Castle write Strategy” waveform, it is difficult to perform sampling of a peak level at a center of the pulse using a relatively simple sample hold circuit. More specifically, a photo detecting mechanism and assistant circuits following to the photo detecting mechanism need to have very high performance with respect to a control band. Accordingly, the cost of the circuits is high.